Devices and methods for offset and similar printing systems

ABSTRACT

A method for producing a half-tone image faithfully representing a continuous-tone original includes a step of forming in the image a pattern of substantially continuous, substantially parallel lines at a frequency of between about 100 and 400 lines per inch, such that the width of the lines at a given point in the image varies in relation to the optical density of at least one color component at a corresponding point in the continuous-tone original. The method may be used for color separations with each separation produced by scanning at a different angle. The method may be implemented using photomechanical or electronic scanning techniques.

This application is a continuation of Ser. No. 08/981,891, filed Nov.23, 1992, now U.S. Pat. No. 5,283,140, which is continuation of Ser. No.08/553,425, filed Jul. 17, 1990, now abandoned.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to devices and methods for offset andsimilar printing systems.

More particularly, the present invention relates to devices and methodsfor screening information from a continuous tone original to produce ahalftone image on a photosensitive layer.

In particular, the invention relates to a novel screen,photo-mechanically or electronically generated, for the preparation ofoffset or similar printing plates to be used in offset or similarprinting of halftone copies of continuous tone originals.

There exists a wide variety of photomechanical screens. Their purpose isto break up the continuous tone information of the original intodiscrete dots, whose size is related to the optical density of theoriginal. The resultant film, upon processing, produces a "dot" image ofthe original.

In the prior art systems all the dots, which are small enough for theeye to reconstruct into a coherent image are of equal opaque opticaldensity while their relative sizes produce the impression of variousgray values. The practical resolution of these screens, and thosegenerated electronically (e.g. via an Electronic Dot Generator Scanner)is usually about 100-200 dots/inch.

In the past, the standard screen utilized an array of symmetrical roundor square dots. The details of the original reproduced through such ascreen are determined by the arbitrary geometrical centers of the dotsin the screen. Further, in the resulting halftone, until 50% dot densityone has black dots on a transparent background. An abrupt jump in thetonal scale occurs at 50%, where the four corners of the square dot joinat 50%. This discontinuity prevents smooth midtone transitions.

To overcome the latter limitation, an elliptical, diamond, multi-dot orstar-like dot pattern is mainly used in modern offset printing. Thesedot shapes allow a smoother tonal gradation than the square dot, in the50% area. Two opposite corners of e.g., the diamond shaped dot, join theadjacent dots first at about 40%, while the other two remaining cornersjoin adjacent dots, near 60%. Since the dots join in two steps, asmoother tonal transformation is achieved. This strategy, of havingadjacent dots join in multiple steps, is used, for example, in theDouble Dot Policrom Screen and the Triplet Dot HRS Beta Screen. Theresult is smoother, but far from ideal, as the abrupt jump in tonalscales is only attenuated to some degree. Aside from these jumps theimage remains discontinuous in the X and Y direction because of thespacing between the dots in both directions which becomes more apparentwith a lower screen ruling.

SUMMARY OF THE INVENTION

In contradistinction to said prior art photomechanical screens andmethods, using the same or involving electronically generating aneffective electronic equivalent thereof, the present invention nowprovides a device for offset and similar printing methods from acontinuous tone original to produce a halftone image on a photosensitivelayer comprising means for screening said information via a first arrayof parallely extending lines of effective minimum optical density and asecond array of parallely extending lines of effective maximum opticaldensity, said arrays being interposed to form a composite array ofsubstantially parallel, spaced-apart, alternating lines of maximum andminimum optical density, said lines delimiting therebetween zones ofeffective graduated optical density, the gradient inside each zoneprogressively varying from low optical density adjacent lines of saidfirst array to high optical density adjacent lines of said second array,the spacing of lines of said first array being between about 100 and 400lines per inch and wherein the resulting thickness of each reproducedline on a resulting developed photosensitive layer, generated byscreening said information via said array, continuously varies as afunction of the density of each information point of the original.

In a preferred embodiment of the present invention there is now provideda photomechanical screen for offset and similar printing methods byscreening information from a continuous tone original to produce ahalftone image on a photosensitive layer, said screen comprising a firstarray of parallely extending lines of minimum optical density and asecond array of parallely extending lines of maximum optical density,said arrays being interposed to form a composite array of substantiallyparallel spaced-apart, alternating lines of maximum and minimum opticaldensity, said lines delimiting therebetween zones of graduated opticaldensity, the gradient inside each zone progressively varying from lowoptical density adjacent lines of said first array to high opticaldensity adjacent lines of said second array, the spacing of lines ofsaid first array being between about 100 and 400 lines per inch andwherein the resulting thickness of each reproduced line on a resultingdeveloped photosensitive layer, generated by screening said informationvia said array, continuously varies as a function of the density of eachinformation point of the original.

The invention also provides a method for generating a halftone imagefrom a continuous tone original comprising exposing a commercial lith orline film to a continuous tone original via a device as hereinbeforedefined wherein the resulting thickness of each resulting line generatedon said developed lith or line film continuously varies as a function ofthe density of each information point of the original.

A preferred embodiment of the present method comprises electronicallyoutputting the information of the original onto an unexposed scannertype film, in an electronic output simulating a screen format,as definedabove, the pattern of this screen being lines spaced 100-400 lines/inchwithout this film being in contact with a standard photomechanicalscreen.

In another aspect of the present invention there is also provided amethod for generating a halftone image from a continuous tone originalcomprising outputting the information of the original onto an unexposedscanner-type film, in an electronic output simulating a screen format,the pattern of this screen in a screen format, the pattern of thisscreen being a first array of parallely extending lines of effectiveminimum optical density and a second array of parallely extending linesof effective maximum optical density, said arrays being interposed toform a composite array of substantially parallel, spaced-apart,alternating lines of effective maximum and minimum optical density, saidlines delimiting therebetween zones of effective graduated opticaldensity, the gradient inside each zone progressively varying from lowoptical density adjacent lines of said first array to high opticaldensity adjacent lines of said second array, the spacing of lines ofsaid first array being between about 400-600 lines per inch anddeveloping in a lith developer containing a hydroquinone developingagent in combination with carbonate, bicarbonate and halide salttogether with sodium formaldehyde sulfoxylate and formaldehyde wherein aresulting thickness of each resulting line generated on said scannertype film continuously varies as a function of each information point ofthe original, but does not decrease in the Dmin region beyond 5 micronswidth even when the generated line becomes segmented.

In U.S. Pat. No. 4,768,101 there is disclosed a method of generating ahalf-tone representation of an image from digital data defining thecolor content of pixels arranged in a series of substantially parallelinput scan lines.

U.S. Pat. No. 4,700,235 discloses a method and apparatus for producinghalf-tone printing forms with screens having arbitrary screen angles andscreen width.

U.S. Pat. No. 4,833,546 discloses a photomechanical apparatus adapted toprint a half-tone picture corresponding to an original continuous tonepicture on the basis of tonal information signals obtained byphotoelectrically scanning the original continuous tone picture.

U.S. Pat. No. 4,547,812 discloses a method and apparatus for forminghigh resolution half-tone images.

U.S. Pat. No. 4,543,613 discloses a method for producing half-tone dotsin a half-tone plate recording apparatus.

As will be realized none of said patents teach or suggest the device andmethod of the present invention.

The present invention provides a screen capable of producing a finalprinted, ink on paper, resolution of 100-400 lines/inch, which generatesa continuous variation in tonal value in one direction, by using ascreen composed of fine lines, 100-400 lines/inch. For example, at 250lines per inch parallel to the direction of the lines, one would say,using prior art concepts and terminology that the segments of a linejoin to generate an effectively continuous line at about 5-6% dotdensity. Perpendicular to the direction of the lines, the adjacent linespartially join above 90% dot density.

According to proposed terminology applicable to the novel concept of thepresent new invention in fact a continuous information straight linefrom the original, parallely aligned and screened via a line of minimumoptical density of the present screening array will be reproduced as acontinuous line.

Similarly in the present screening array the width of each line segmentis determined by the density of each information point of the original,each such segment having its center along a line of minimum opticaldensity and extending continuously perpendicularly to the direction ofthe lines on both sides up to the adjacent bracketing lines of maximumoptical density.

Since the "dots" join in two steps outside the operational limits ofwhat would be considered in prior art terms as 5%-90% dot density thetonal transformation is, intrinsically, smooth across the whole tonalrange, simply because there are no jumps in tonal scale, within thisrange, in the direction of the lines. The line screen creates a halftoneimage by continuously changing the width of the parallel lines,comprising the screen pattern. Thus, unlike "dot" based prior artphotomechanical or electronic screens, it renders the originalcontinuous tone, in a continuous format, in one direction. For monocolorwork, there remains a discontinuity in the direction perpendicular tothe lines. However, even in this direction, the image remains black on awhite background, until above about 95% dot density. There is no abruptchange to white on black, which is the major cause of disturbance to theeye.

In multicolor printing, where different angles are employed for each ofthe separations (e.g., here for the line screen, the optimal angles are,45°, 90°, 105° and 165°), this discontinuity is attenuated, and theresult appears to approach a symmetrical continuous tone to theobserver.

A possibly more graphic way of explaining the present invention is tostate that while the prior art of which applicants are aware is based onsymmetrical dot, diamond, elliptical or multi-dot two-dimensional arraysof varying optical density which could be analogized to the peaks andvalleys of an egg tray, the present invention is based on parallelyextending lines with effective varying optical density of alternatingparallely arranged valleys and ridges analogous to a corrugated roof.

In preferred embodiments of the present invention it has been found thatusing a screening means wherein the distance between lines of said firstarray are between about 200 and 300 lines per inch with normal rapidaccess or lith development, will generate a halftone, that with anyplate, ink or press, will produce a finer "continuous tone" reproductionof an original continuous tone, than comparable commercially availablescreens. This is because, intrinsically, these prior art screens arediscontinuous in both the X and Y directions, whereas the line screen ofthe present invention is continuous in the direction of the line.

As will be seen with reference to FIG. 1 described hereinafter, below100 lines per inch the eye will perceive the discrete lines of the linescreen pattern and thus while Beta Screen Corp. produces a specialeffect straight line contact screen having a line density of 55lines/inch, such a lower range cannot be used for the purposes of thepresent invention.

According to calculations as set forth in example 1 below, much beyond400 lines/inch, the width of the lines approaches 5μ for 50% dotdensity. Below 50%, well above the Dmin value, the line has to be lessthan 5μ, which is too small for graphic art films, so the result is 100%transparent. Over 50%, well below Dmax value, the distance between linesis less than 5μ. Again, graphic art films cannot handle it, and silverimage will be joined over this distance, creating solid black. Thus formost uses contemplated the effective upper range should not exceed 400lines per inch.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in connection with certain preferredembodiments with reference to the following illustrative figures so thatit may be more fully understood.

With specific reference now to the figures in detail, it is stressedthat the particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention, the description taken with the drawings making apparent tothose skilled in the art how the several forms of the invention may beembodied in practice.

In the drawings:

The file of this patent contains at least one color photograph. Copiesof this patent with color photograph(s) will be provided by the Patentand Trademark Office upon request and payment of necessary fee.

FIGS. 1a, 1b and 1c are representations of dot arrays of prior artphotomechanical screens.

FIG. 1d is a representation of a dot array at a density of 25 and 85dots per inch, respectively.

FIG. 1e is a representation of a line array at a density of 25 and 85lines per inch.

FIG. 2 is a contact of a resolution target produced using prior art filmand chemistry.

FIG. 3a is a contact positive produced according to the presentinvention.

FIG. 3b is a comparison contact positive produced using a prior art dotscreen.

FIG. 4 is a developed scanner film produced using a line density of 500lines/inch, according to the invention.

FIG. 5a is a color photo of a color print produced according to thepresent invention.

FIG. 5a-1 is an enlargement by about ×8 of the eye of FIG. 5a.

FIG. 5b is a comparison color photo of a color print produced using anelectronically generated dot screen according to the prior art.

FIG. 5b-1 is an enlargement by about ×8 of the eye of FIG. 5b.

FIG. 6 is a color photo of a color print produced according to thepresent invention.

FIG. 6a is an enlargement by about ×8 of the eye of FIG. 6.

Referring now to the drawings in detail, FIGS. 1a, 1b and 1c have merelybeen provided as background for a better visual understanding of thestate of the prior art in which photomechanical contact screen employedarrays of square dots (FIG. 1a), elliptical dots (FIG. 1b) and the HRSTriplet Dot.sup.(R) (FIG. 1c) of Beta Screen Corp.

FIGS. 1d and 1e provide a comparison of a standard screen format and aline pattern at a dot and line density of 25 and 85 per inchrespectively. As can be seen already at a density of 85 lines per inchthe line pattern appears more "continuous tone" than the standardformat.

It follows from the above that a new type of screen pattern, imposed onthe film, comprising lines spaced between 100-400 to the inch where thethickness of the line is a function of the density in the original, willprovide a continuous tone reproduction, superior to other screens, aswill be shown with reference to further figures appended hereto.

EXAMPLE 1

Standard lith film (Agfa DLD510) was exposed through a resolution targetand developed in standard lith developer (Agfa G 110C). Segment A ofFIG. 2 shows 6μ width black continuous lines on white background. Linesless than 6μ width effectively disappear. White lines on a blackbackground start to appear continuous from a width of about 8μ. Atwidths smaller than about 8μ, the lines join each other. Good qualityprinting requires a minimum density (Dmin) of 5-6% in the highlightregion and a maximum density (Dmax) of 90-95% in the shadow region. Fora 250 lines/in ruling, 5-6% density corresponds to a 5-6μ line width,according to the following calculation: 25 mm=1 in, 1 mm=1000μ.

This calculation generates the following Table: ##EQU1##

As shown in FIG. 2 a screen ruling of about 250 lines per inch canprovide continuous lines even in the highlight region. In the shadowregion, the halftone requires only 85-90% density to provide the 90-95%density printed result, because of the limitations of the printingtechnology at a 250 lines/in resolution. An 8μ width of white lineswhich corresponds to more than 90% density black remains, as shown inFIG. 2 continuous; i.e. the lines do not join up or close up.Consequently, from the highlight to the shadow region, the image is madeup, respectively of continuous and well separated lines. This analysisthus teaches that a ruling in the range of about 250 lines/inch withstandard reprographic film and chemistry will produce a superiorhalftone as described, e.g., hereinafter with reference to FIG. 6.

FIG. 2 also shows that a ruling less than 250 line/in producescontinuous, well separated, parallel lines. At high resolution, e.g.,500-600 lines/in, the usable, reproducible and printable linescorrespond to too high a Dmin and too low a Dmax, to obtain a goodquality image with standard lith film and chemistry since all lowerdensities do not appear and all higher densities create a solid blackimage. With a special lith chemistry, as described in U.S. Pat. No.4,598,040, even this high resolution produces good values of Dmin andDmax, even though the lines are not absolutely contiguous, as describedin example 3 hereinafter.

EXAMPLE 2

FIG. 3 (girl)

An image was exposed through a 55 line/in screen (Beta Screens) onto astandard lith film (Agfa 812) and processed in standard lith developer(Agfa G 90P). This was reduced about 55%, on Agfa 812 film and developedin Agfa G 90P generating a ruling of 100 lines/in. This resultant "linescreened" image was exposed onto a standard lith (Agfa 812) anddeveloped in standard lith developer (Agfa G 90P). The resulting blackand white contact positive using Agfa-Gevaert Litex paper developed inD-11 is shown in FIG. 3a. As a Control, the same continuous tone imageused above was exposed through a conventional dot screen (Agfa Gevaert)with a ruling of 100 dots/in, onto a standard lith film (Agfa 812) anddeveloped in a standard lith developer (Agfa G 90P). The resultingcontact positive on Agfa-Gevaert Litex paper developed in D-11 is shownin FIG. 3b. This demonstrates that the "line screen" image produces asmoother transition of tones (i.e. a better continuous tone) relative toa standard screen, in this domain of screen ruling.

EXAMPLE 3

FIG. 4 (Beach Scene)

An Itek continuous tone scanner (Model 200) outputs at 500 lines/in.FIG. 4 shows the results of this 500 line/in output onto a standardscanner film (Agfa 812) using a special lith developer, containing ahydroquinone developing agent in combination with carbonate, bicarbonateand halide salt together with sodium formaldehyde sulfoxylate andformaldehyde as described in U.S. Pat. No. 4,598,040, the teachings ofwhich are incorporated herein by reference. (At this resolution, a colorseparation can be made without requiring adjustment of angles, tominimize Moire).

Careful inspection of FIG. 4 reveals in the highlight area, relativelycontinuous lines, comprising short line segments in excess of 5μ widespaced close enough to each other to give an impression of continuityalong their axis, and in the shadows, longer line segments which tend to"close up" partially, near Dmax. As will be realized from this Figure,in the highlight region the width of the line segments are 5-10μ butthey generate these low densities by varying the length of the segmentsat the expense of spacing between the segments on the same lines of theproduced halftone. In the shadow region the width of the lines are40-45μ and continuous. The high resolution of 500 lines per inch or 50μdistance between adjacent lines is obtained because the low densityregion, e.g., 5% is not comprised of solid black lines of 2.51μ widthwhich is not printable, but rather of line segments 5-10μ wide withsuitable spacing between the segments in the produced halftone.

EXAMPLE 4

(FIG. 5)

A color separation set (cyan, magenta, yellow, black) was prepared,where each color was produced electronically as a line screen, eachaligned 30 degrees from each other, onto a standard red sensitivescanner film (Anitec) and developed in standard rapid access developer(Anitec), at a ruling of 254 lines/in using a LaserPaint.sup.(R)Macintosh.sup.(R ) /Linotronic 300.sup.(R) System. Contacts of each ofthe 4 films in the set were made using standard lith film (Agfa) 812)and developing in a special lith developer, as described in U.S. Pat.No. 4,598,040. The resulting color print is shown in FIG. 5a. Thecontrol was made using a normal 2-directional electronically generatedscreen format with a ruling of 254 dots/in the screen, angles beingdealt with in the standard way. The resulting color print is shown inFIG. 5b. Note, that in this case, this color separation generates 8lines effectively (2 for each color) whose relative angles have beenconsidered to minimize Moire vis only 4 lines for the line screen.Offset plates were prepared from these color separations, and they wereprinted up in the standard way. Comparison of FIG. 5a with FIG. 5bdemonstrates that the advantages of a "line screen" for color is evengreater than that for black and white.

EXAMPLE 5

(FIG. 6)

A color separation set (cyan, magenta, yellow, black) was prepared,where each color was produced electronically as a line screen, aligned30 degrees from each other onto a standard red sensitive scanner film(Anitec) and developed in standard rapid access developer (Anitec), at aruling of 250 lines/in, using a LaserPaint^(R) Macintosh^(R) /Linotronic300^(R) System. Offset plates were prepared from these colorseparations, and they were printed up in the standard way. The resultingcolor print is shown in FIG. 6, which confirms the analysis in Example1, i.e., at about 250 lines/in, using standard scanner film/chemistrysystem, a reasonably continuous line is generated in the highlights, abroader and really continuous line in the middletones, without getting a"closing up" (discontinuous) effect (i.e. the lines remain parallelwithout joining) in the shadows, until the Dmax region is approached.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrative embodiments andthat the present invention may be embodied in other specific formswithout departing from the spirit or essential attributes thereof. Thepresent embodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed is:
 1. A method for producing a color image faithfullyrepresenting a continuous-tone color original comprising the stepsof:(a) producing a first half-tone image corresponding to a first colorcomponent of the continuous-tone color original, by forming in saidfirst half-tone image a pattern of substantially continuous,substantially parallel lines at a frequency of between about 100 and 400lines per inch, such that the width of said lines at a given point inthe image varies in relation to the optical density of said first colorcomponent at a corresponding point in the continuous-tone original, saidsubstantially parallel lines in said first half-tone image having afirst line frequency, said substantially parallel lines in said firsthalf-tone image defining a first direction; (b) producing a secondhalf-tone image corresponding to a second color component of thecontinuous-tone color original, by forming in said second half-toneimage a pattern of substantially continuous, substantially parallellines at a frequency of between about 100 and 400 lines per inch, suchthat the width of said lines at a given point in the image varies inrelation to the optical density of said second color component at acorresponding point in the continuous-tone original, said substantiallyparallel lines in said second half-tone image having a second linefrequency substantially equal to said first line frequency, saidsubstantially parallel lines in said second half-tone image defining asecond direction rotated from said first direction; (c) producing athird half-tone image corresponding to a third color component of thecontinuous-tone color original, by forming in said third half-tone imagea pattern of substantially continuous, substantially parallel lines at afrequency of between about 100 and 400 lines per inch, such that thewidth of said lines at a given point in the image varies in relation tothe optical density of said third color component at a correspondingpoint in the continuous-tone original, said substantially parallel linesin said third half-tone image having a third line frequencysubstantially equal to said first line frequency, said substantiallyparallel lines in said third half-tone image defining a third directionrotated from both said first direction and said second direction; and(d) combining said first, said second and said third half-tone images inregistration.
 2. A method according to claim 1, further comprising thestep of producing a fourth half-tone image corresponding to a fourthcolor component of the continuous-tone color original, said step offorming said fourth half-tone image including forming in the image apattern of substantially continuous, substantially parallel lines at afrequency of between about 100 and 400 lines per inch, such that thewidth of said lines at a given point in the image varies in relation tothe optical density of said fourth color component at a correspondingpoint in the continuous-tone original, said substantially parallel linesin said fourth half-tone image having a fourth line frequencysubstantially equal to said first line frequency, said substantiallyparallel lines in said fourth half-tone image defining a fourthdirection, rotated from each of said first direction, said seconddirection and said third direction.
 3. A method according to claim 2,wherein each of said first direction, said second direction, said thirddirection and said fourth direction are rotated from all other saiddirections by substantially integer multiples of 30°.
 4. A methodaccording to claim 1, wherein said step of forming in the image apattern of substantially parallel lines is performed using aphotomechanical screen.
 5. A method according to claim 1, wherein saidstep of forming in the image a pattern of substantially parallel linesis performed using an electronic scanner.